Power generating systems employing fuel cells have been gaining much attention in recent years as a source of more efficient and cleaner from of energy. As known, a fuel cell is an electrochemical energy conversion device which produces direct current (DC) electricity from an external supply of fuel (such as hydrogen) and oxygen, as opposed to the internal energy storage supply of a battery.
Typically, a fuel cell power source is implemented in a “stack” of multiple fuel cells in a power generating system. A “balance-of-power system” (BOPS) is connected to the fuel cell stack for providing the fuel and oxygen. The power generating system also includes a battery for providing power to the load during transient moments when the load demand changes suddenly as the load goes to a new operating condition. The fuel cell stack generally cannot handle or react immediately to rapid changes in the current demand. Rapid changes in the current load demand, in the long term, are also harmful to the reliability of the fuel cell stack. After the new operating condition has been reached, the battery is phased out and the fuel cell stack provides the power to the load. The fuel cell stack also charges the battery during this time.
Conventional power generating systems employing a fuel cell stack/battery arrangement includes a dedicated power conditioner that is connected between the fuel cell stack and the load, and another dedicated power conditioner connected between the battery and the load. The two separate dedicated power conditioners add to the cost, volume and power loss of the power generating system.
Moreover, each of the two separate power conditioners in the conventional power generating system are specifically designed for rated power, around which the power conditioners have a very high efficiency. Therefore, in the steady state, when the load demand drops, the power conditioners, while providing lower output power operates at a low efficiency. This lead to inefficient operation of the power generating system.
Conventional fuel cell power systems operate at a constant fuel utilization in the steady state. However, the fuel cell stack operates at its optimal efficiency at a certain range of the fuel utilization in the stack, which depends on the operating fuel (hydrogen) flow rate into the stack. As such, the fuel cell stacks operate sub-optimally.